The broad objective of our work is to understand the neural regulation of cardiac conduction and excitation. The specific objectives of this proposal are to study the dynamic interaction of transient and stea state vagal and sympathetic stimulation on dromotropic and inotropic cardiac responses, and the dynamic three-way interaction of heart period and activity of both autonomic divisions on these responses. We shall also study the influence of atrial fibrillation on the autonomic control of AV conduction. In particular, the autonomic influence on the compensatory pause, and on the ventricular rate, and the contribution of the baroreflex to ventricular function will be studied with atrial fibrillation. Finally, we shall study the time dependencies of cardiac responses to brief bursts and continuous trains of vagal stimuli, and provide data relative to the possible mechanisms of these primary and secondary transient effects.